Process for galvanizing a ferrous metal article

ABSTRACT

IN ASENDZIMIR-TYPE PROCESS FOR CONTINUOUSLY HOT-DIP GALVANIZING A STEEL TRIP PROVIDED WITH SPECIAL PHYSICAL PROPERTIES BEFORE GALVANIZING WHICH AVOIDS DESTRUCTION OF THE SPECIAL PHYSICAL PROPERTIES DURING THE GALVANIZING PROCESS, A STEEL STRIP IS PLACED WITH A THIN SURFACE CATING OF A NON-FERROUS METAL WHICH FORMS AN OXIDE AND WHOSE OXIDE ISREDUCIBLE AT A TEMPERATURE BELOW 10500*F., AND PREFERABLY BELOW ABOUT 900*F., AND THE COATING OF NONFERROUS METAL ISOXIDIZED IN AN OXIDIZING ATMOSPHERE TO FORM AN OXIDE FILM, THE OXIDE FILM IS REDUCED TO THE METALIC FORM BY HEATING THE STRIP IN A REDUCING ATMOSPHERE TO A MAXIMUM TEMPERATURE OF 1050*F. AND PREFERABLY NO HIGHER THAN ABOUT 900*F., AND THEN THE STRIP IS IMMERSED IN A HOT-DIP GALVANIZING BATH WHILE THE NON-FERROUS METAL ISMAINTAINED IN THE METALLIC FORM. THE NON-FERROUS METALS FOUND MOST USEFUL FOR SURFACE COATING OF THE STEEL STRIP ARE NICKEL, COPPER, AND COMBINATIONS OF NICKEL AND COPPER. THE NON-FERROUS METAL COATING IS PREFERABLY APPLIED TO THE STRIP BY PLATING EITHER ELECTROLYTICALLY OR BY NONELECTROLYTIC DISPLACEMENT TO PROVIDE A COATING HAVING A WEIGHT RANGING BETWEEN ABOUT 10 AND 1000 MG./FT.2 WITH APREFERRED COATING WEIGHT OF BETWEEN ABOUT 25 AND 50 MG.0FT.2.

United States Patent @ftice Filed June so, 1971, Ser. No. 158,225

1m. (:1. B4411 1/34; C23c 1/02 U.S. Cl. 117-51 Claims ABSTRACT OF THEDISCLOSURE In a Sendzimir-type process for continuously hot-dipgalvanizing a steel strip provided with special physical propertiesbefore galvanizing which avoids destruction of the special physicalproperties during the galvanizing process, a steel strip is placed witha thin surface coating of a non-ferrous metal which forms an oxide andwhose oxide is reducible at a temperature below 1050 F., and preferablybelow about 900 F., and the coating of nonferrous metal is oxidized inan oxidizing atmosphere to form an oxide film, the oxide film is reducedto the metallic form by heating the strip in a reducing atmosphere to amaximum temperature of 1050 F. and preferably no higher than about 900F., and then the strip is immersed in a hot-dip galvanizing bath whilethe non-ferrous metal is maintained in the metallic form. Thenon-ferrous metals found most useful for surface coating of the steelstrip are nickel, copper, and combinations of nickel and copper. Thenon-ferrous metal coating is preferably applied to the strip by platingeither electrolytically or by nonelectrolytic displacement to provide acoating having a weight ranging between about 10 and 100 mg./ft. with apreferred coating weight of between about and 50 mgr/fif The presentinvention relates to a process of hot-dip galvanizing ferrous metalarticles, including hot-dip coating a steel sheet with zinc and withalloys of zinc contain ing aluminum, magnesium, or aluminum andmagnesium and the like alloying metals which are suitable for forming aprotective zinc base coating.

In galvanizing a ferrous metal strip or sheet material in accordancewith a Sendzimir-type hot-dip coating process which eliminates the useof fluxes, a clean ferrous metal sheet free of scale is oxidized toprovide a thin uniform iron oxide surface film which is free of alloxidizable impurities. Since the oxide surface is not coated by ahot-dip galvanizing bath when immersed therein, however, the sheet mustbe passed through a reducing atmosphere heated to an elevatedtemperature to reduce the iron oxide film to metallic iron. And, whilethe substantially pure iron surface is protected against reoxidadon, thestrip is immersed in a hot-dip galvanizing bath to provide an adherentgalvanized coating.

While the foregoing Sendzimir hot-dip galvanizing process forms anadherent uniform protective metal coating on a ferrous metal strip andthe surface thereof is readily oxidized when exposed to an oxidizingatmosphere, it is necessary to subject the ferrous metal strip havingthe iron oxide film on the surface thereof to a reduction-heat treatmentat an elevated temperature of between about 1500 F. and 1800 F. andgenerally to about 1700" F. in a reducing non-oxidizing atmosphere sothat the iron 3,726,703 Patented Apr. 10, 1973 oxide surface film willbe reduced to metallic iron in order to provide a surface to which thezinc will adhere. It is also advisable to cool the strip whilemaintaining the strip in a reducing non-oxidizing atmosphere to justabove the temperature of the hot-dip galvanizing bath, which isgenerally at a temperature of about 860 F., before immersing the stripin the galvanizing bath.

Heating a steel strip to a temperature ranging between about F. and F.,however, is well above the temperature which causes a substantial changein the physical properties of the steel where the strip has beenprovided with special physical properties before the strip is galvanizedin a Sendizimir-type galvanizing process. For example, if a steel sheetis provided with a martensitic structure before being coated in aSendzimir-type galvanizing process, the sheet will have practically noneof the desired martensitic structure after passing through the reductionstep wherein the strip is heated to a temperature of about 1500 F. orabove. It is preferable not to heat the martensitic strip substantiallyabove 900 F. during the galvanizing process, if the martensiticstructure is to be largely retained.

It is therefore the object of the present invention to provide animproved Sendzimir-type hot-dip galvanizing process which makes itunnecessary to heat a ferrous metal strip appreciably above thetemperature of a hot-dip galvanizing bath when preparing the surface ofthe strip for coating in a hot-dip galvanizing bath.

It is another object of the present invention to provide an improvedSendzimir-type hot-dip process for galvanizing a steel strip having amartensitic structure which greatly reduces loss of martensiticstructure in the strip during the galvanizing process.

It is a further object of the present invention to provide an improvedSendzimir-type hot-dip process for galvanizing a steel strip having afull hard crystalline structure which avoids significantly changing thephysical properties of the strip during the galvanizing process.

It is still another object of the present invention to provide animproved Sendzimir-type hot-dip galvanizing process which eliminates theexpense and time required to heat the ferrous metal strip substantiallyabove the temperature of a galvanizing bath and then cool the strip downto about about the temperature of the galvanizing bath when preparing asteel strip for the hot-dip galvanizing bath.

It is a further object of the invention to provide an improvedSendzimir-type galvanizing process which can be operated moreeconomically and at a faster line speed.

Other objects of the present invention will be apparent from thedetailed description and claims to follow when read in conjunction withthe accompanying drawing comprising a process flow chart showing thesteps of the process.

In achieving the foregoing objects in accordance with the presentinvention a clean scale-free ferrous metal article, such as a steelstrip or sheet, is provided with a thin metallic surface coating of anon-ferrous metal which readily forms an oxide which is reducible to themetallic form at a temperature substantially below 1500 F., andpreferably below a maximum temperature of 1050 F., since a markedreduction or complete loss of the special physical properties of aferrous metal strip, such as Where the strip has been treated to providefull hard mechanical properties, results when the strip is heated to atemperature range between 1500 F. and 1800 F., in the regularSendzimir-type galvanizing process. The non-ferrous metals whose oxidehas the foregoing properties and which are suitable for surface coatinga steel strip in accordance with the present invention include nickel,copper, silver and cobalt. Nickel or copper and combinations thereofhave been found most suitable from a cost standpoint, and because thecoatings form oxides which are reducible to the metallic form at arelatively low temperature (i.e., below 900 F.) and at a temperatureclose to the melting point of the galvanizing bath. In addition thinoxide coatings of nickel, copper and combinations thereof aresufficiently ductile to remain on the strip during the bending of thestrips while passing through a continuous coating line. Othernon-ferrous metals which otherwise would be suitable but which have aboiling point below the temperature of the galvanizing bath or aboiling'point below the reduction-heat treating temperature required forreducing the oxide of the thin surface coating of non-ferrous metal tothe metallic form are not preferred for use in accordance with thepresent invention. The non-ferrous metal surface coating can be formedby any suitable means or process and can be extremely thin withoutimpairing the usefulness in the present invention. The non-ferrous metalsurface coating on the ferrous metal can have a coating weight rangingbetween about and 100 rng./ft. and preferably between about 25 and 50mg./ft.

In accordance with the present process the ferrous metal article havinga surface coating of a suitable nonferrous metal is oxidized to form asurface oxide film by exposing to oxidizing conditions by any suitableprocedure while maintaining the temperature of the article as low aspossible and below about 1050" F. and preferably below about 900 F.Thereafter, and immediately before hot-dip galvanizing, the articlehaving the oxidized surface film of non-ferrous metal is passed througha reducing zone which heats the article to a maximum temperature ofabout 1050" F., and preferably below about 900 F., to effect completereduction of the oxide film to the metallic state. Thereafter, and whilethe surface coating of the non-ferrous metal remains in the metallicstate under the protection of a non-oxidizing or reducing atmosphere,the article is cooled, if necessary, to just above the galvanizing bathtemperature and the ferrous metal article is passed through a hot-dipgalvanizing bath of any type to provide a uniform protective hot-dipgal- I vanized surface coating thereon.

The hot-dip galvanizing process of the present invention is preferablycarried out continuously on a continuous coating line of the type shownin the flow sheet in the accompanying drawing, wherein a ferrous metalsheet material, such as a steel strip 10, is continuously supplied to acleaning apparatus 11 as an endless strip. In the cleaning apparatus 11the sheet 10 is preferably first cleaned in an alkaline cleaning bath ofa type conventionally used to remove surface contamination and preparethe ferrous metal strip for coating. Thereafter, the strip 10 is passedthrough a hot water rinse tank 12 and treated in a conventional acidpickling bath 13 which can comprise dilute hydrochloric or sulfuric acidto remove surface rust and scale before coating. The strip 10 free ofscale, surface oxides, and surface contamination is passed through acold water rinse bath 14 and is in condition to receive a surfacecoating of a non-ferrous metal of the type previously described hereinand which forms an oxide film reducible to the metallic state at atemperature below about 1050 F. and preferably below 900 F. to avoidcausing drastic changes in the physical properties of the ferrous metalbase material having the special physical properties.

The steel strip 10 is preferably provided with the thin surface coatingof a suitable non-ferrous metal by continuously passing the strip 10through a metal plating bath 15. In the preferred embodiments of thepresent invention the non-ferrous metal applied as a thin coating to thestrip 10 is nickel or copper or a combination of both nickel and copperin any proportion and is preferably applied electrolytically or by meansof a chemical dip in a preferred coating weight ranging between about 25mg./ft. and 50 mg./ft. by coordinating the length of time the strip isimmersed in the plating bath with the current density or bathcomposition and the like plating variables. In a continuous nickel orcopper plating line the dwell time of the strip in the plating bath canrange between about 4 and 15 seconds. The thickness or weight of themetal surface coating is preferably such that the coating is not sothick that it is brittle or appreciably interferes with the formation ofa diffusion layer between the ferrous metal base material and thehot-dip coating material.

Acid nickel plating baths are preferred for electroplating a thin nickelfilm, and a typical bath for nickel electroplating has the followingcomposition:

Nickel sulfate oz./gal 44 Nickel chloride .oz./gal 6 Boric acid oz./ga15 pH l.54.5.

Bath temperature F -14O Current density amp./ft. 20-100 An acid copperplating bath can be used, but instead of electroplating a coppercoating, a special electroless copper plating bath is used with a coppersulfate to sulfuric acid having a weight ratio of about 1 to 10 andimmersion time and condtions regulated to prevent forming excessivelythick and uneven coatings of metallic copper. A suitable acid copperelectroless plating bath developed for the present process has thefollowing composition:

Copper sulfate-1 oz./ gal. Sulfuric acidl0 mL/gal. Bath temperatureRoomtemperature.

Alkaline copper plating solutions such as those disclosed in any metalplating handbook can be used to provide a suitable thin electroplatedmetallic copper coating which when treated in accordance with thepresent invention results in forming a high quality firmly adherenthot-dip coating.

Following hot water rinsing in the bath 16 and air drying the chamber 17the strip 10 having the thin non-ferrous metal metallic coating on bothsurfaces is passed through a heating and oxidizing zone 18 to form athin film of oxide on the surface of the non-ferrous metal coating. Theoxidizing zone 18 can be of any type which will oxidize the surface ofthe non-ferrous metal coating, such as an oxidizing chamber whichcontains an atmosphere which is oxidizing to the metal coating andheated to a temperature sufiicient to oxidize the metal surface coating.The temperature within the chamber or zone 18, of course, can be as highas about 1200 F. or even higher and as low as about 250 F., depending onthe dwell time of the strip 10 in the zone 18 and the nature of thenonferrous metal coating. The oxidizing zone 18 can also comprise aflame directed onto the surface of the coated strip 10 just before thestrip enters the reducing zone 19. The thickness of the oxidized filmpreferably is such that the oxide film does not exceed that which can becompletely reduced to the metallic state while being passed continuouslythrough the reducing zone 19. The oxide film can be thin and consistsessentially of surface oxidation of the non-ferrous metal coating.

Following surface oxidation, the sheet 10 immediately before immersionin a hot-dip coating bath 20 is passed through a reducing zone 19capable of reducing the oxide film of nonferrous metal which preferablycomprises a chamber containing a heated reducing non-oxidizingatmosphere which heats the sheet to a temperature not exceeding about1050 F. and preferably below about 900 F. (i.e. between about 660 F. and880 F.) to effect complete reduction of the oxide film to the metallicform, thereby forming a film which is readily coated by the moltenhot-dip galvanizing bath. The reducing atmosphere can be formed ofhydrogen in diluted form (i.e., 20% hydrogen-80% nitrogen) ordisassociated ammonia or the like, with care being taken to maintain thedew point sufficiently low to prevent the atmosphere in the reducingzone 19 becoming oxidizing to the metal surface coating. In aSendzimir-type continuous galvanizing line the strip may require up to3-4 minutes to pass through the reduction-heat treating chamber.

While the surface coating remains in the fully reduced form under aprotective reducing non-oxidizing atmosphere, and while the strip is ata temperature preferably only slightly above the temperature of thehot-dip galvanizing bath 20 (i.e., at a temperature of about 860 F.),the strip 10 is passed through the hot-dip galvanizing bath 20 to form afirmly adherent hot-dip galvanized coating over the entire surface ofthe strip 10.

Tests were conducted on an experimental continuous coating line whichclosely approximates actual continuous galvanizing coating lineconditions and in which a continuous strip of the material as indicatedin the Table I was hot-dip coated by passing the strip through each ofthe steps of the present process, including applying nickel or copper incoating weights of about 10-100 mg./ft. and immersing in a conventionalhot-dip zinc coating bath travelling at the line speed indicated inTable I and during which the strip was heated to the maximumtemperature, as indicated in Table I. Control runs were also made underidentical conditions but without application of a thin coating of nickelor copper. The resulting galvanized product was examined forcompleteness and uniformity of zinc coverage and adherence of the zinccoating. The following Table I shows the results observed:

The foregoing Table I shows that excellent zinc coverage is providedWhen a ferrous metal strip has a nickel or copper coating appliedthereto and heated to a peak temperature above 550 F. and below about900 F. during a Sendzimir-type galvanizing process.

It will be evident that with the present process steel strips which havebeen box-annealed prior to hot-dip galvanizing can be reduction-heattreated in the galvanizing process at a temperature sufliciently low tomaintain the very soft properties of box-annealing Without endangeringproper coverage by the molten zinc. Also, steels which have full-hardproperties produced by tandem rolling can also be hot-dip galvanized toprovide excellent zinc coverage without heating above the temperaturewhich causes a substantial change in the crystal structure thereofresulting in a serious loss of the full-hard or annealed properties ofthe steel. In like manner, high strength steels, such as martensiticsteel strips can be effectively continuously hot-dip galvanized at amaximum strip temperature range of about 850 F.-900 F so that the highstrength mechanical properties of the steel are substantially retainedafter hot-dip galvanizing.

The process of the present invention, in addition to making it possibleto hot-dip galvanize ferrous metal sheets which have been treated toprovide special physical or mechanical properties not possessed byconventional galvanizing steel sheet material without destroying thespecial physical or mechanical properties, also significantly reducesthe expense of heating and cooling the sheet in a Sendzimir-typegalvanizing line. And, since an excellent zinc base coating can behot-dip applied in accordance with the present invention when heatingthe strip in the oxide reducing zone to a temperature of about 660 F.,still further savings in heat and time would be achieved, if a hot-dipgalvanizing bath having a melting point below 850 F.-875 F. were used.

We claim:

1. A process of hot-dip galvanizing a ferrous metal article whichcomprises: applying on a surface of a ferrous metal article to behot-dip galvanized an adherent surface coating of a non-ferrous metalwhich forms an oxide when heated in an oxidizing atmosphere to atemperature below about 1050 F. and whose oxide is reducible in areduction-heating zone at a temperature below about l050 F., oxidizingsaid coating of non-ferrous metal to form a surface oxide film withoutheating said article above about 1050 F., heating said article havingsaid surface oxide film of non-ferrous metal in a reductionheating zonecontaining a reducing non-oxidizing atmosphere to a temperature belowabout 1050 F. to effect complete reduction of said surface oxide film toa metallic form, and immersing said article in a hot-dip galvanizingbath while said non-ferrous metal is maintained in said metallic form toprovide an adherent galvanized coating on said ferrous metal article.

2. A process of hot-dip galvanizing as in claim 1, wherein said articleis an endless steel sheet which has been treated to provide specialphysical properties not possessed by conventional galvanizing steelsheet material, and wherein said strip is heated to a temperature whichdoes not exceed about 900 F. during said oxidizing and metal is selectedfrom the group consisting of nickel,

copper and a combination of nickel and copper in any proportion.

4. A process as in claim 2, wherein said coating of non-ferrous metalhas a weight between about 10 mg./ft. and mg./ft.

5. A process as in claim 3, wherein said coating of nonferrous metal hasa coating weight between about 25 and 50 mg./ft.

6. A process of hot-dip galvanizing as in claim 3, wherein said sheethaving said surface oxide film of nonferrous metal thereon is heated insaid reducing nonoxidizing atmosphere at a temperature between about 660F. and 880 F.

7. A process as in claim 1, wherein said metal article is an endlesssteel strip which prior to applying said coating of non-ferrous metalthereto is treated to increase the hardness thereof above that ofconventional galvanizing steel.

8. A process as in claim 7, wherein said endless strip of sheet steel isprovided with martensitic structure before application of said coatingof non-ferrous metal, and wherein the maximum temperature to which saidstrip is heated during the said process is about 880 F., whereby thegalvanized martensitic strip retains a major proportion of saidmartensitic structure.

9. A process as in claim 1, wherein said metal article is an endlesssteel strip which prior to applying said coating of non-ferrous metalthereto is annealed to provide full soft properties therein.

10. A process as in claim 1, wherein a metallic copper coating isapplied to said article by immersing said article in an electrolesscopper plating bath for a period of between about 4 seconds and 15seconds, with said copper plating bath containing copper sulfate andsulphuric acid in an approximate weight ratio of 1 to 10.

References Cited UNITED STATES PATENTS ALFRED L. LEAVITT, PrimaryExaminer I. A. BELL, Assistant Examiner US. Cl. X.R.

11750, 71 M, 130 R, 131, 114 A

